1. Field of the Invention
This invention relates to a spray gun that atomizes an adhesive agent, paint or other liquid and sprays it on the target object, and is specifically characterized by its needle valve mechanism.
2. Description of the Related Art
Conventionally, spray guns are widely used to spray adhesive agents, paint, and other types of liquids in order to coat the target object. Known as spray guns of that type are “air spray guns” that use a jet stream of air to atomize the liquid during spray coating and “airless spray guns” that spray liquid as a mist by adding pressure to the liquid in order to spray coat the target object. With such spray guns, those that have a needle valve and use that needle valve to adjust the volume of liquid flow are generally used.
In particular, a conventional spray gun is equipped with a tubular nozzle that contains the outlet for the liquid in its tip and a needle valve that is inserted inside that nozzle. A fixed-side restricting section is provided inside the nozzle on its inner circumference, and an aperture is formed around the needle valve for the liquid to pass through between the needle valve and the restricting section. That aperture is enlarged along with the retracting movement of the needle valve and the aperture is narrowed along with its forward movement. A valve portion is provided on the tip of the needle valve and serves as a movable-side restricting section that changes the amount of liquid that reaches the outlet through that aperture. In this way, the forward and retracting movement of the needle valve is used to adjust the volume of liquid flow coming from the outlet, and it is possible to spray coat the target object with that liquid by making the compressed air that is emitted from the air jetting port collide with the liquid that is discharged from the outlet, or by atomizing the liquid through pressure that is added to the liquid.
The main parts of an example of an air spray gun that atomizes liquid with a jet stream of air for spray coating are concretely illustrated in FIGS. 14A to 14C. A nozzle 200 of an air spray gun is shown in FIG. 14A, with a liquid passage 202 having an annular cross section being formed between the nozzle 200 and a needle valve 210. The nozzle 200 includes a round outlet 204 at its tip where liquid is discharged, and then, to the rear of the outlet 204, includes a female tapered surface 206 having a circular cross section. The female tapered surface 206 forms fixed-side restricting section 208 at a part of its tip end. The needle valve 210 is a narrow-shaft (needle-shape) component with a circular cross section, its tip is specified as male taper-shaped valve portion 212, and the outer peripheral surface of this valve portion 212 is specified as a male tapered surface 214 whose diameter becomes progressively smaller toward its tip.
The needle valve 210 is made movable on the central axis of the nozzle 200 in the axial direction (the left-right direction in FIG. 14A) and, through its forward movement, the aperture with a ring-shaped cross section between the valve portion 212 and the fixed-side restricting section 208, which serves as a valve seat, is narrowed in the radial direction. And, at its forward end, by having the valve portion 212, specifically the male tapered surface 214, come in contact with the restricting section 208 in the axial direction, that aperture in the radial direction is closed over the entire the circumference. In other words, the liquid passage 202, which extends to the outlet 204, can be blocked and closed. Meanwhile, the aperture with a ring-shaped cross section is produced between the valve portion 212 and the restricting section 208 along with the retracting movement of the needle valve 210, and the aperture is then further enlarged in the radial direction by the continued retracting movement of the needle valve 210. The amount of liquid that passes through that aperture is thus adjusted in accordance with the change in the size of that aperture. In other words, the amount of liquid that is discharged from the outlet 204 is changed. Here, a cap 216 is provided on the periphery of the tip of the nozzle 200, separated from it by a designated distance, thus forming an air passage 218 with an air jetting port 220 located between this cap 216 and the nozzle 200.
With this air spray gun, liquid is drawn out of the outlet 204 by the air jet stream from the air jetting port 220. In other words, the liquid is discharged from the outlet 204. Furthermore, that discharged liquid is atomized into a mist through a collision with the compressed air. And, as is schematically shown in FIG. 15, the particles of the atomized liquid ride the air jet stream and are spray coated onto the target object. The example illustration in FIG. 15 shows a case wherein liquid is spray coated onto a small target object W (described later).
Here, with a conventional air spray gun 222, wherein the valve portion 212 comes in contact with the restricting section 208 of the nozzle 200 in the axial direction at the forward end of the needle valve 210 to close the liquid passage 202, there is the problem that, depending on the dimensional tolerance of the restricting section 208 and the valve portion 212 when the liquid passage 202 is closed, microscopic gaps are generated between the valve portion 212 and the restricting section 208. This kind of problem with a conventional air spray gun is indicated, for example, in Japanese Unexamined Patent Publication No. JP-A-2008-012403. Another problem with microscopic gaps being generated due to the fiber contained in the liquid getting caught between the valve portion 212 and the restricting section 208 when the liquid passage 202 is closed is also indicated in Japanese Examined Utility Model Publication No. JP-Y-6-046523.
There are also the problems of the contact between the restricting section 208 of the nozzle 200 and the valve portion 212 of the needle valve 210 resulting in uneven wear and damage to the restricting section 208 and the valve portion 212, both of which are made of metal, when the liquid passage 202 is closed, or by the abrasion that is generated when the liquid passage 202 is opened and closed, and of microscopic gaps thus being formed. That type of trouble is indicated, for example, in U.S. Publication No. US 2002/0195505.
And, if gaps of this kind are generated, liquid will leak from those gaps. Such leaking liquid can coagulate at the outlet 204, partially blocking the outlet 204, or it will drip, coagulate and then block a portion of the air jetting port 220 as shown in FIG. 16A. This phenomenon readily occurs especially when that liquid is something that disperses microscopic solid particles through the use of a solvent, because those solid particles can easily clump together. If that situation occurs, the direction that the liquid is sprayed, as shown in FIG. 16B, will diverge irregularly on a course that is inclined from the expected and suitable direction of application, and liquid will thus be spray coated in a direction that differs from that originally intended. As a result, for example, when spray coating liquid onto a small target like the target object W, there will be an insufficient amount of liquid that is actually applied onto the coated surface of the target object W.
In addition to the above, the following problems are also generated with a conventional air spray gun. For example, when spray coating liquid onto the small target object W, the appropriate amount of liquid coating is actually only a minute amount. In this case, in order to apply that proper minute amount of liquid, the aperture between the restricting section 208 of the nozzle 200 and the valve portion 212 of the needle valve 210 becomes extremely minute (for example, about 0.06 mm), as shown in FIGS. 14B and 14C.
When spray coating liquid using this kind of minute aperture, it is easy for liquid to clog up such a small opening. If liquid is sprayed when that kind of clogging exists, it will not be possible to spray coat the liquid appropriately onto the target object W. In that case, it is unavoidable to retract the needle valve 210 in order to enlarge the aperture between the restricting section 208 and the valve portion 212 and spray the liquid.
However, in that case, if the liquid is being spray coated onto the target object W at close range, an excessive amount of liquid will be applied to the target object W and, as a result, an uneven coat will be produced, or there will be a problem of the liquid that is applied to the target object W dripping. In order to prevent that, it is necessary to secure a large amount of separation between the air spray gun and the target object W, and thus keep the position from which the liquid is applied onto the target object W at a distance.
In that case, however, the mist flow of the liquid from the air spray gun will be widely dispersed before it reaches the target object W. In other words, the range that the liquid is sprayed in will become unnecessarily large. Due to that, the amount of liquid that passes by the target object W, scatters, and is thus not applied to the target object W, increases. This liquid that disperses and scatters without coming in contact with the target object W thus turns into a loss and results in a large drop in yield. And, that causes an increase in the cost that is required for that liquid.
With the aim of resolving the problem of leakage occurring due to dimensional tolerance, etc., when the valve portion of the needle valve comes in contact with the nozzle's restricting section in the axial direction at the forward end of that needle valve in order to close the liquid passage, US 2002/0195505 indicated an O-ring (78) being retained on the nozzle side and the valve portion of the needle valve coming in contact with that O-ring (78) at the forward end of the needle valve so that, in addition to closing that passage through the contact between the restricting section on the nozzle side and the valve portion of the needle valve, it would also be closed through the contact between that O-ring and the valve portion. In other words, a dual closure mechanism (double seal) is implemented. However, since the valve portion of the needle valve in US 2002/0195505 is in contact with the O-ring (78), when the amount of liquid to be applied is a minute amount, the amount of flexure in the O-ring (78) affects the amount of that coating, so the problem of a drop in precision in the adjustment of the volume of liquid flow is generated when spray coating with a minute amount of liquid.
Although the explanation above used an air spray gun as an example, the same problems are inherent in a hydraulic airless type of spray gun as well.